1. Field of the Invention
The present invention relates to a low boron amorphous alloy and a process for producing the same, specifically to a low boron-containing Fe--Si--B base amorphous alloy which achieves improved magnetic properties together with scattering reduction. The term "low boron" is here intended to define an Fe--Si--B alloy containing about 6-10 atomic percentage of boron.
2. Description of the Related Art
Various Fe--B--Si base alloy compositions have excellent soft magnetic properties. An amorphous alloy composition comprising 80 to 84 atomic percent (at %) of iron, 12 to 15 at % of boron and about 6 at % of silicon is disclosed in U.S. Pat. No. 4,300,950 of Chen, Luborsky et al. Further, an alloy comprising 77 to 80 at % of iron, 12 to 16 at % of boron and 5 to 10 at % of silicon is disclosed in U.S. Pat. No. 5,370,749.
Thus, almost all Fe--Si--B base amorphous alloys which have so far been known have a content of boron of more than 10 at %.
This is because boron is important to prevent crystallization of the alloy. The higher the boron content, the stronger the amorphous formability of the alloy, and the better the alloy thermal stability.
Magnetic properties of those Fe--Si--B base amorphous alloys having a boron content of 10 at % or less have been inferior in core loss and flux density, as compared with those having a boron content of more than 10 at %.
Accordingly, reports on Fe--Si--B base amorphous alloys having a boron content of more than 10 at % are very scarce. Reported more often are alloys containing carbon as a material for improving stability toward change on standing, and resistance to crystallization in Japanese Unexamined Patent Publication No. 57-145964 and Japanese Unexamined Patent Publication No. 58-42751. Also reported are alloys containing Mn as a material for improving surface-treating properties (Japanese Unexamined Patent Publication No. 61-136660) and alloys containing Cr as a material for improving castability (Japanese Unexamined Patent Publication No. 58-210154).
In addition thereto, the characteristics of low boron alloys are lacking for reasons already described above.
It is described in Japanese Unexamined Patent Publication No. 4-333547 that a reduction of core loss in a high frequency range of electrical steel is a requisite for improvement of a core loss by controlling plate thickness. However, the high frequency range used in that publication is a very high frequency range such as 100 kHz, 200 KHz, 500 KHz or 1 MHz. It is known that a large part of a core loss consists of an eddy current loss in such a high frequency range, and it is also known that eddy current loss can be reduced by decreasing plate thickness.
In contrast with this, it is known that in a commercial frequency area as applied to the present invention, some optimum value of plate thickness is present for minimizing core loss in the case of an Fe--Si--B base amorphous alloy. Reduction of the plate thickness to the optimum value or less rather increases the total core loss because of increased hysteresis loss.
Further, it is reported in Japanese Unexamined Patent Publication No. 62-192560 that the space factor is elevated by controlling ribbon roughness. Core loss and flux density are affected by reduction of ribbon roughness, which facilitates transfer of magnetic domain walls and therefore decreases hysteresis loss but increases eddy current loss since coarsening of the magnetic domain takes place.